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Technical Article
Parallel & Series Operation
CONTENTS
•
Parallel Operation
•
Series Operation
This is only recommended with power converters
specifically designed for parallel connection. A
general comment is that it is much lower cost and
causes far fewer problems to use a single power
converter correctly rated for the application rather
than two or more in parallel. However, there are
power converters which feature master slave parallel
operation. These units are intended for modular
expansion schemes and fault tolerant parallel
redundant power systems. Where power converters
are overload protected by constant current limiting
simple parallelling of the outputs can work to an
acceptable standard. Output voltages must be set
the same (as precisely as possible). In a two unit
system the unit with the slightly higher output
voltage will reach its current limit and the voltage will
drop to equal that of the other unit.
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Parallel Operation
This converter will then supply the remaining current demanded by the load. So regulation can never be better than the difference between the output
voltage settings of the two converters, and one unit will always be operating in current limit, therefore above its rating. Where current
limits are adjustable to below maximum rating simple parallelling is satisfactory if the degradation of regulation can be tolerated. To improve load
current sharing, equal series resistors can be used (See Fig. 1).
Figure 1. Current sharing resistors used to parallel two converters
For the best results the wiring resistance must also be exactly balanced. Small differences in the output voltage settings of the converter output still
creates considerable current unbalance. In the example illustrated, the load is 5V at 2A. Converter output voltage settings are 5V but if they are
unequal by 0.1V, the current out of balance from the nominal 1A is 0.5A. This requires that each unit is individually rated at 1.5A. It is clearly not a cost
effective method of proving 5V 2A of stabilized power. Also the 100m series resistors degrade the regulation to worse than 2%.
It should be bourne in mind that as the output power of the supply rises, the voltage drop and power dissipated rise. The power rating of the resistor
should be carefully calculated. At higher currents, the series resistance should be reduced to perhaps 25-30mΩ.
In critical applications where continuous operation is essential, parallel redundant power systems are often specified. The system has to keep running
even when a power unit fails. Current sharing is not such an important criterion since each power unit must be rated to supply the total load. But to
enable both units to be continuously monitored for faults it is advisable that some measure of current sharing takes place. Both units are then always
operating. Isolating series diodes which are continuously rated at the full load current allow either power converter to continue operation unaffected by
a fault in the other. Matching the forward resistance of the diodes and balancing the wiring resistance helps with the current sharing. However, these
series impedance degrade the regulation. Some power converters, which are specifically designed for use in fault tolerant systems allow remote sense
downstream of the parallelling diodes to maintain full regulation at the load. In a parallel redundant scheme one of the power converters could be
replaced by a battery followed by a DC-DC converter to provide a no-break power system in the event of main supply failure (See Fig. 2)
Figure 2. Isolated diodes used for parallel redundant connections of two converters
2
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Series Operation
Most power converters can be operated in series if they have overload limit characteristics, constant current or constant power circuits. With some
switching converters series operation is prohibited because one unit upsets the feedback regulation system of the other. With linear and switched
mode units using foldback current limiting, lock out at switch ON can occur because of the different ramp up times of two units in series. Care must
be taken not to exceed the safe working voltage at the outputs of converters in series. This may be considerably lower than the dielectric strength test
voltage which is a short-term test between outputs and ground. The output ripple of converters in series is additive but this of course does not change
the value of ripple expressed as a percentage of total output voltage. To protect each output from the reverse voltage applied by the other unit in the
event of load short circuits, reverse biased diodes are used. It is common practice to include these protection diodes in laboratory power supplies.
Figure 3. Outputs connected in series showing reverse voltage protection diodes
3
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